1. Field of the Invention
This invention relates to a control system for hybrid vehicles having a plurality of prime movers.
The disclosure of Japanese Patent Application No. 2004-120773 filed on Apr. 15, 2004 including specification, drawings and claims is incorporated herein by reference in its entirety.
2. Discussion of the Related Art
One example of a hybrid vehicle is disclosed in Japanese Patent Laid-Open No. 2002-225578. The disclosed hybrid vehicle has a drive unit, in which an engine, a first motor generator and an output member are connected through a power distributing mechanism, and the output member and a second motor generator are connected through a transmission. Accordingly, power of the engine is distributed to the output member and to the first motor generator through the power distributing mechanism. The torque is added to or subtracted from the output member by controlling the second motor generator. Additionally, it is possible to improve the fuel economy by controlling the engine speed in line with the optimum fuel economy curve.
According to the hybrid drive unit disclosed in Japanese Patent Laid-Open No. 2002-225578, a transmission capable of being interchanged between a high gear stage and a low gear stage is arranged between the second motor generator for carrying out mainly a torque assist and the output shaft. Therefore, the output torque of the second motor generator is transmitted to the output shaft in accordance with the speed change ratio of the transmission. On the other hand, the transmission carries out the speed change operation by applying or releasing an engagement device, therefore, the torque is not transmitted temporarily at a transient time of the speed change operation.
For this reason, in case a downshifting is carried out by the transmission when the vehicle is run by inertia, the output torque of the second motor generator connected to the input side of the transmission is lowered to suppress an abrupt rise in the speed of the second motor generator. Then, the output torque of the second motor generator is raised to restore a creep torque upon completion of the speed change. In this case, the torque acts on the output shaft at a termination of the speed change so that the output shaft is twisted as a result. If the driving force of the vehicle is raised by the torsional torque simultaneously with the restoration of the creep torque which is lowered during the speed change, the driving torque becomes temporarily excessive. As a result of this, shocks arising from the speed change operation become prominent.